This book constitutes the thoroughly refereed
post-conference proceedings of the 9th International Conference on Quantum
Interaction, QI 2015, held in Filzbach, Switzerland, in July 2015.



The 20 papers together with 2 invited keynotes presented
in this book were carefully selected from 27 submissions. Quantum Interaction
has developed into an emerging interdisciplinary area of science combining
research topics in mathematics, physics, psychology, economics, cognitive
science, and computer science.